Recent reading about axlotls plunged me down a rabbit hole of developmental biology regarding the preservation of juvenile or larval traits into adulthood. The wiki page about this contains a lot of questionable claims about how this applies to human development, but there’s no doubt that neoteny pops up all over the place.

Coincidentally I’ve been keeping a few species of Boraras, which are a recently-described genus of fish that are very similar to the well-known Rasboras but have their bits slightly rearranged and are comically tiny. ‘Boraras’ is an anagram of ‘Rasbora’ which is intended to indicate that they’re an anatomical anagram as well.

Here’s a Boraras, B. merah:


And, here’s another one, B. urophthalmoides:

Boraras uraphthalmoides

Both of those photos are of full-grown adults, about 2cm long.

Just now I’m raising a tank full of baby Rasbora volcanus. Adult volcanus are ‘full sized’ for a Rasbora, 6cm or so, and they adults are nondescript, shiny minnows with no particular markings:


But, their young look just like Boraras, including the black smudge on the side:


I thought I’d found a perfect case of neoteny, with Boraras preserving the markings of their juvenile ancestors. Disappointingly, this article asserts that Boraras are something else.

The anatomical structure of miniaturised cyprinids can vary greatly, and there are two principle ‘groupings’ with some species possessing intermediate features to some degree. The first contains those fishes which though small are essentially proportionally dwarfed versions of their larger relatives, e.g., Barboides, Microdevario, Microrasbora, Boraras, etc.

The other includes those in which anatomical development stops at a point where adult still resemble a larval form of their larger ancestor, i.e., Danionella and Paedocypris.

The latter are usually referred to as ‘developmentally truncated’ or ‘paedomorphic’ and are thought to have evolved via a process known as ‘progenetic paedomorphosis’ i.e. paedomorphosis brought about by accelerated maturation. They typically exhibit a simplified skeletal structure along with species-specific morphological peculiarities.

Presumably the difference is that there were incremental ever-smaller ancestors to Boraras rather than a sudden few-generation jump from full-sized ancestor to paedomorph. I’m still puzzled by the black patch on the side… I’ll have to raise a few more species of Rasbora to see if that’s a consistently juvenile trait or just a fluke of this particular species.

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